However, a simple Beta distribution will struggle to capture the morphology of the true population if a significant fraction of binary black holes have low (\(\lesssim 0.01\) ) natal spins, which is anticipated to be the case if angular momentum transport in massive stars is efficient [1]}, [2]}.
The mixing parameter \(\lambda _0\)  controls the fraction of black holes merging with negligible spin.
We assume that the spin of the primary black hole in a binary is independent from the spin of the secondary black hole.
